In recent years, we have developed many spontaneously metastasizing mammary adenocarcinomas in the highly inbred strain of W/Fu rats. Their immunological and biochemical characteristics have been well defined and found to have numerous similarities with human cancer, including metastasizing patterns and soluble antigen-shedding property. Utilizing this tumor system, we hope to learn the precise mechanisms of metastasis by studying the interaction between the tumor cell surface and the host immune system with respect to, 1) the functional capacity of T-lymphocytes; 2) the role of macrophages; 3) the role of B-lymphocytes; 4) the role of complements and antigen-antibody complexes; and 5) the interrelationship between lysosomal enzymes, glycosyltransferases, and tumor associated antigens released by the metastasizing tumor cells.